1. Field of the Invention
The present invention relates to a laser processing method of performing laser processing to a single crystal substrate such as a silicon (Si) substrate, sapphire (Al2O3) substrate, silicon carbide (SiC) substrate, and gallium nitride (GaN) substrate.
2. Description of the Related Art
As well known in the art, in a semiconductor device fabrication process, a functional layer is composed of an insulating film and a functional film is formed on the front side of a silicon (Si) substrate, and a plurality of semiconductor devices such as ICs and LSIs are formed like a matrix from this functional layer, thus obtaining a semiconductor wafer having the plural semiconductor devices. The plural semiconductor devices are partitioned by a plurality of crossing division lines formed on the front side of the semiconductor wafer. The semiconductor wafer is divided along these division lines to obtain the individual semiconductor devices as chips.
Further, in an optical device fabrication process, an optical device wafer is provided by forming an optical device layer composed of an n-type nitride semiconductor layer and a p-type nitride semiconductor layer on the front side of a sapphire (Al2O3) substrate, silicon carbide (SiC) substrate, or gallium nitride (GaN) substrate. The optical device layer is partitioned by a plurality of crossing division lines to define a plurality of regions where a plurality of optical devices such as light emitting diodes and laser diodes are respectively formed. The optical device wafer is cut along the division lines to thereby divide the regions where the optical devices are formed from each other, thus obtaining the individual optical devices as chips.
As a method of dividing a wafer such as a semiconductor wafer and an optical device wafer along the division lines, there has been tried a laser processing method of applying a pulsed laser beam having a transmission wavelength to the wafer along the division lines in the condition where the focal point of the pulsed laser beam is set inside the wafer in a subject area to be divided. More specifically, this wafer dividing method using laser processing includes the steps of applying a pulsed laser beam having a transmission wavelength to the wafer from one side of the wafer along the division lines in the condition where the focal point of the pulsed laser beam is set inside the wafer to thereby continuously form a modified layer inside the wafer along each division line and next applying an external force to the wafer along each division line where the modified layer is formed to be reduced in strength, thereby dividing the wafer into the individual devices (see Japanese Patent No. 3408805, for example).
As another method of dividing a wafer such as a semiconductor wafer and an optical device wafer along the division lines, there has been put into practical use a technique including the steps of applying a pulsed laser beam having an absorption wavelength to the wafer along the division lines to thereby form a laser processed groove along each division line by ablation and next applying an external force to the wafer along each division line where the laser processed groove is formed as a break start point, thereby breaking the wafer along each division line (see Japanese Patent Laid-open No. Hei 10-305420, for example).